In recent years, thin lightweight display devices such as liquid crystal displays (LCD) and plasma display panels (PDP) have attracted much attention for their applicability in computer and television image display. In particular, the responsiveness, wide viewing angle, and suitability for large-screen application of PDPs have resulted in widespread moves in industry and research to secure a market for PDPs.
A PDP is formed from a front glass substrate having a plurality of display electrodes and a back glass substrate having a plurality of address electrodes. The front and back glass substrates are arranged parallel to and facing one another, and a plurality of barrier ribs are provided in a stripe-pattern therebetween. Phosphor layers in the order red (R), green (G), and blue (B) are formed in the gap (“rib gap”) between adjacent barrier ribs, and the gaps are filled with a discharge gas.
Image display in the PDP is achieved when ultraviolet light, which is emitted as a result of a discharge generated when a drive circuit is used to apply a voltage to the electrodes, strikes the phosphor particles of the phosphor layers and excites them to emit visible light.
The phosphor layers are commonly formed using a screen-printing method, in which the rib gap between adjacent ribs is filled with a phosphor ink, and the ink is then baked. However, this screen-printing method is not readily applicable in the manufacture of PDPs in which the width of the rib gap has been reduced in response to demands in recent years for higher definition image display. In a full specification (1920×1125 pixels) 42-inch high definition (HD) PDP, for example, the rib pitch is a fine 0.1 mm to 0.15 mm, and when a thickness of the barrier ribs is taken into account, a narrow rib gap of 0.08 mm to 0.1 mm remains within which to apply the ink. Since the phosphor ink conventionally used in screen-printing has a high viscosity running into tens of thousands of centipoises, accurately applying the phosphor ink in a narrow rib gap at a high speed is not easily achieved.
Alternative methods for forming the phosphor layers include a photoresist film method, a photoresist ink method, and an inkjet method.
According to the photoresist film and ink methods, either a film is embedded or an ink is applied in the rib gap between adjacent barrier ribs. The film and the ink are both formed from a photosensitive ultraviolet resin that includes RGB phosphors. After the film is embedded or the film is applied, the areas of film or ink that will form the phosphor layers are exposed and developed, while the unexposed film or ink is washed away. According to both these methods, the phosphor layers can be formed in the rib gaps with a reasonable degree of accuracy, even when the barrier ribs are finely pitched.
However, in addition to manufacturing complications resulting from the embedding/applying, exposing, developing, and washing having to be conducted sequentially for each of the three colors RGB, there is the problem of the colors easily becoming mixed. Moreover, because of the relatively high cost of the phosphors and the difficulties involved in collecting phosphors washed away during the washing process, the photoresist film and ink methods are expensive to implement.
In comparison, according to an inkjet method as disclosed in unexamined patent application publications 53-79371 and 8-162019 filed in Japan, an ink formed from a phosphor material and an organic binder is discharged under pressure though a plurality of nozzles of an ink application apparatus while scanning up and down to apply the ink in the desired pattern.
This inkjet method allows for the phosphor ink to be applied accurately within a desired rib gap, and therefore provides a simple and cost effective means of forming the phosphor layers that dispenses with the exposure and washing processes required in the photoresist methods.
The phosphor ink conventionally used in the inkjet method is a mixture of an organic binder (ethyl cellulose, acrylic resin, or polyvinyl alcohol, etc.), a solvent (terpineol, butyl carbitol acetate, etc.), and phosphor particles.
However, the comparatively high dielectric constant of the organic binder and the solvent results in the ink becoming charged from the shearing stress that occurs when the ink passes through the tubing and nozzle parts of the ink application apparatus. As a result, the ink flows discharged from the plurality of nozzles react with each other, causing dispersion in the delivery of the ink. Consequently, the phosphor ink is applied and adheres in an uneven manner. In order to improve brightness in a PDP it is important that the phosphor layers be applied evenly to the walls and base of the gap between adjacent barrier ribs, although this is difficult to achieve with the conventional inkjet method described above.
Apart from phosphor ink, the inkjet method can also be employed to apply inks that include particles used in forming structural layers of the PDP other than the phosphor layers. Examples of such particles include silver particles used in forming silver electrodes and dielectric glass particles used in forming dielectric layers. However, since these alternative inks also include the solvents and organic binders described above, there remains the problem of the uneven adhesion of the ink resulting from dispersion in the ink delivery as well as reductions in the solvent concentrations in the ink.